1. Field of the Invention
The present invention relates to an aerogel type platinum-ruthenium-carbon catalyst and a method for manufacturing the same. More particularly, the invention relates to an aerogel type platinum-ruthenium-carbon catalyst, maintaining a long-term high catalytic activity, manufactured by a sol-gel process and supercritical drying process, and a method for manufacturing the same. The invention also relates to a direct methanol fuel cell employing the aerogel type platinum-ruthenium-carbon catalyst as an anode catalyst.
2. Description of the Related Art
Fuel cells are devices for producing electricity by converting chemical energy of fuels into electrical energy via electrochemical reactions. Such fuel cells may be divided into high temperature types, medium temperature types and low temperature types, according to operating temperatures. Particularly, low temperature fuel cells such as Polymer Electrolyte Fuel Cells (PEFCs) use platinum or platinum alloy catalysts, to ensure that electrode catalysts have sufficient catalytic activity even at low temperatures.
Fuel cells commonly utilize hydrogen as a fuel. Typical electrochemical reactions that take place in a fuel cell are represented in the following Equation 1.
Anode: H2xe2x86x922H++2exe2x88x92
xe2x80x83Cathode: 1/2O2+2H++2exe2x88x92xe2x86x92H2O
Full cell reaction: H2+1/2O2xe2x86x92H2Oxe2x80x83xe2x80x83[Equation 1]
Once reactants are fed continuously, the difference in potential energy between the anode and the cathode makes an electromotive force to produce current. Pure hydrogen as a fuel, however, has several disadvantages. Cells using hydrogen require a high manufacturing cost. In addition, it is difficult to store and transport hydrogen, the fuel. Therefore, attempts to use other substances as a fuel for cells have been made. One example of them is a Direct Methanol Fuel Cell (DMFC) using methanol. When using methanol as a fuel, cells have advantages of smaller size, easier fuel supply, and reduced problems of recycling and waste disposal.
DMFCs have the same constituents as those used in PEFCs which use hydrogen as a fuel. DMFCs also use mainly platinum or platinum alloy catalysts as anode catalysts. In such cells, protons and electrons are generated by a chemical reaction as in Equation 2 below.
H2O+CH3OHxe2x86x92CO2+6H++6exe2x88x92xe2x80x83xe2x80x83[Equation 2]
The protons generated at this time migrate to the cathode via the electrolyte between the anode and the cathode. The protons react with O2 on the platinum catalyst, as in Equation 3 below.
3/2O2+6H++6exe2x88x92xe2x86x923H2Oxe2x80x83xe2x80x83[Equation 3]
The full cell reaction occurring at this time is as in Equation 4.
CH3OH+3/2O2xe2x86x92CO2+2H2Oxe2x80x83xe2x80x83[Equation 4]
However, DMFCs have problems of shorter life of the cells and lower energy density, as compared to cells using hydrogen as a fuel. DMFCs require a large amount of expensive precious metals to enhance a catalytic activity of the anode. In addition, the catalysts are degraded by poisoning due to CO generated during the electrochemical reactions.
Meanwhile, though platinum is generally known as an anode catalyst for use in DMFCs and PEFCs, platinum itself has a problem upon its use. The reason is that CO is strongly adsorbed by the surface of platinum, poisoning the catalyst, thereby dramatically deteriorating the catalytic activity. CO is present as an un-oxidized product of methanol in DMFCs, while CO being present in residual quantities among raw materials in PEFCs. Trials to solve such a problem have been made by alloying platinum with ruthenium, tin, rhenium, molybdenum, etc. (T. Freelink, W. Vischer, J. A. R. Van Vcen, Electrochim. Acta 39: 1871, 1994; U.S. Pat. No. 6,232,264). Such alloying has two advantages of minimizing CO-poisoning and reducing the amount of platinum used. Especially, platinum-ruthenium alloy catalysts are known to be the best for methanol oxidation, compared to other platinum alloys, and are now commercially available.
Platinum-ruthenium alloys are commonly manufactured using a melting process at high temperature (H. A. Gasteiger, N. Markvic, P. N. Ross Jr., E. J. Cairns, J. Phys. Chem. 97:12020, 1993). Liquid-phase reduction using a reducing agent is also available. Research on depositing a platinum-ruthenium alloy on a carbon support is ongoing (V. Radmilovic, H. A. Gasteiger, P. N. Ross Jr., J. Catal. 154:98, 1995). According to the above depositing technique, active metals are highly dispersed on a carbon support which has good electro-conductivity and a large surface area, thereby reducing the amount of platinum used per unit area, while improving cell efficiency. Although this technique is ideal, it is not yet reported that carbon-supported catalysts prepared via impregnation or colloid route (M. Watanabe, M. Uchida, S. Motoo, J. Electroanal. Chem., 229:395, 1987) show greatly improved performance, as compared to unsupported platinum-ruthenium alloy catalysts.
Meanwhile, for catalysts, inorganic aerogels with ultrahigh-porosity prepared via a sol-gel process and supercritical drying process, starting with a metal alkoxide, are known. Currently, active research on methods of synthesizing carbon aerogels is ongoing. To synthesize such a carbon aerogel, an organic gel prepared via polycondensation of organic monomers is subjected to supercritical drying, thereby making an organic aerogel, followed by carbonization (R. W. Pekala, J. Mater. Sci., 24:3221, 1989; U.S. Pat. No. 4,997,804). Further, there was a report on results of carbon aerogel synthesis, where metals are uniformly dispersed by adding a small amount of transition metals such as chrome, molybdenum, tungsten, iron, cobalt, nickel, etc. (F. J. Maldonado-Hodar, C. Moreno-Castilla, J. Rivera-Utrilla and M. A. Ferro-Garcia, Stud. Sur. Sci. Catal., 130:1007, 2000).
However, these studies failed to find optimal amounts of added solvents, or ideal conditions for supercritical drying including a temperature and pressure.
To date, platinum-ruthenium alloy catalysts exhibit the best anode catalyst performance. Despite this, such platinum-ruthenium alloy catalysts are problematic in terms of application of expensive precious metals. Accordingly, there is a need to maximize dispersion of a platinum-ruthenium alloy over a carbon support, enabling reduction of amounts of platinum or ruthenium used, thereby ensuring a great economic benefit.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide an aerogel type platinum-ruthenium-carbon catalyst, which is manufactured by a sol-gel process and supercritical drying process, having an electrode activity and durability superior to catalysts manufactured by conventional methods, and a method for manufacturing the same. It is another object of the present invention to provide a direct methanol fuel cell employing the aerogel type platinum-ruthenium-carbon catalyst as an anode catalyst.
To accomplish the above and other objects, there is provided an aerogel type platinum-ruthenium-carbon catalyst in accordance with the invention, which consists of platinum, ruthenium and carbon, in the form of aerogel having a number of pores, prepared by drying while maintaining a microporous structure.
Preferably, the catalyst contains 5 to 70% by weight of platinum and ruthenium, and the remainder is composed of carbon. An atomic ratio of platinum to ruthenium is 1/4 to 4/1.
The method for manufacturing the aerogel type platinum-ruthenium-carbon catalyst in accordance with the invention comprises the steps of: a first step of adding metal salts and a base catalyst to a solution of organic gel materials; a second step of adding a solution of basic amine to the solution prepared at the first step, and adjusting pH of the resulting solution, thereby forming a sol; a third step of aging the sol at 40 to 90xc2x0 C. for 1 to 20 days after sealing, thereby forming and stabilizing a gel; a fourth step of removing a solvent from the gel via a supercritical drying process, thereby making a type of aerogel; and a fifth step of carbonizing the aerogel in an inert atmosphere, followed by hydrogen reduction.
Preferably, a solution of the organic gel materials may contain resorcinol-formaldehyde, and the solvent is water.
Preferably, a solution of the organic gel materials may contain melamine-formaldehyde, and the solvent is water.
Preferably, a solution of the organic gel materials may contain catechol-formaldehyde, and the solvent is water.
Preferably, if the solvent is water, after the third step of forming a gel, water in the gel may be displaced by methanol, prior to carrying out the supercritical drying process at the fourth step.
Preferably, a solution of the organic gel materials may contain phloroglucinol-formaldehyde, and the solvent is alcohol.
Preferably, the solution prepared at the first step may be adjusted to pH 5 to 8 by adding basic amine at the second step.
Preferably, the aerogel may be carbonized at 500 to 1200xc2x0 C. in an inert atmosphere, followed by hydrogen reduction at 200 to 1000xc2x0 C. at the fifth step.
The direct methanol fuel cell according to the invention employs the aerogel type platinum-ruthenium-carbon catalyst as an anode catalyst.